Hi,

Sending this to the ia64 list, because that is so far the only platform
I have tested on, and because the patch may be more likely to have real
applications on ia64 systems.

I have been looking at different implementations of unmapping and page
table freeing recently. As a consequence, I came to notice that the
vast majority of L2 cache misses on ia64 (and probably all
architectures) in an unmapping workload comes from the line:
	pte_t ptent = *pte;
In zap_pte_range, ie. walking the bottom level page table pages.
I should qualify that - that is the case when the page tables
aren't in cache - this does not apply to a simple lmbench fork/exit
test for example.

Anyway, I tried prefetching a line ahead of the one we're currently
working in, and put the prefetching into zap_pte_range, and
copy_pte_range (which does a similar pte walk to set up page tables
on fork()).

microbenchmark results are pretty good - but I wonder if anyone might
have a real-world use for it?

After applying the recent freepgt patchset from Hugh (on lkml), the
time to fork+exit a process mapping 64GB of address (32MB of page
tables) is 0.471s. With the prefetch patch, this drops to 0.357s.